Voltage regulators have been applied extensively in various electronic products as power supplies. In state-of-art voltage regulators, in order to prevent load from being damaged due to voltage spike of the output in transients, voltage droop function is adapted for diminishing the voltage spike on the load in transients. FIG. 1 schematically shows a conventional voltage regulator 100 with voltage droop function, in which driver 104 switches transistors 106 and 108 coupled in series between input voltage Vin and ground GND in response to pulse width modulation (PWM) signal provided by control circuit 102. Thereby, inductor current IL is generated to charge output capacitor Co so that output voltage Vo is produced. In the control circuit 102, from the voltage drop across current sense resistor Rs due to the inductor current IL flowing therethrough, current sense circuit 110 produces current sense signal
                              Ix          =                                    IL              ×              Rs                        K                          ,                            [                  EQ          ⁢                      -                    ⁢          1                ]            where K is the equivalent resistance of the current sense circuit 110. The current sense signal Ix passes through droop resistor RADJ and produces load line droop voltageVdroop=Ix×RADJ.  [EQ-2]Because of virtual ground, the voltage on pin 116 intends to be equal to reference voltage Vref, and therefore the output voltage will beVo=Vref−Vdroop.  [EQ-3]Error amplifier 112 generates error signal EA from the difference between its inverting and non-inverting inputs, and PWM comparator 114 compares the error signal EA with ramp signal Vramp to determine the PWM signal for the driver 104. From the equations EQ-1, EQ-2 and EQ-3, it is known that the output voltage Vo of the regulator 100 will decrease as the inductor current IL increases.
FIG. 2 schematically shows another conventional voltage regulator 200 with voltage droop function, which comprises control circuit 102, driver 104, transistors 106 and 108, current sense circuit 110, error amplifier 112, PWM comparator 114, and current sense resistor Rs as well. However, the reference voltage Vref is coupled to the non-inverting input of the error amplifier 112 via the droop resistor RADJ and pin 116, and the output of the current sense circuit 110 is coupled to the non-inverting input of the error amplifier 112. When the current sense signal Ix passes through the droop resistor RADJ, load line droop voltage Vdroop is produced as described in the equation EQ-2. The inverting input of the error amplifier 112 is coupled to the output Vo, and thereby the output voltage Vo follows the equation EQ-3 due to virtual ground. Consequently, according to the equations EQ-1, EQ-2 and EQ-3, it is known that the output voltage Vo of the regulator 200 will decrease as the inductor current IL increases.
In a voltage regulator with droop function, the load line droop signal proportional to the output current is sensed by the current sense resistor. Unfortunately, the resistance of an ordinary resistor is a function of temperature, so that the load line is also the function of temperature. Consequently, incorrect operations may happen because the control circuit 102 provides incorrect PWM signal due to the incorrect current sense signal Ix caused by the temperature coefficient of the current sense resistor Rs. In order to prevent mal-operations by the control circuit 102 resulted from temperature variations, the droop resistor RADJ with proper negative temperature coefficient is chosen to be positioned in the vicinity of the current sense resistor Rs to compensate the voltage variations caused by temperature changes on the current sense resistor Rs with positive temperature coefficient. Nevertheless, resistors with negative temperature coefficients are not ordinary resistors and thereby are more expensive. In addition, in order to position the droop resistor RADJ nearby the current sense resistor Rs, the conductive wire between the pin 116 and droop resistor RADJ is lengthened, which makes the pin 116 tend to be affected by switching noises. Moreover, there is always a distance between the resistors RADJ and Rs, and hence the temperature changes in the resistors RADJ and Rs are different, thereby introducing inaccurate compensation to the current sense signal Ix. Furthermore, the position of the droop resistor RADJ in one voltage regulator may be different from that in another, and thereby various resistors with different negative temperature coefficients have to be prepared for the droop resistor RADJ in applications of various voltage regulators.